Buggy code:
@ SuppressWarnings ( { "unchecked" } ) public final class Parser { private static final int DEFAULT_TODOLIST_CAPACITY = 16 ; private final char [ ] input ; private DoubleStack < AbstractStackNode , AbstractNode > [ ] todoLists ; private int queueIndex ; private final Stack < AbstractStackNode > stacksToExpand ; private DoubleStack < AbstractStackNode , AbstractNode > stacksWithTerminalsToReduce ; private final DoubleStack < AbstractStackNode , AbstractNode > stacksWithNonTerminalsToReduce ; private EdgesSet [ ] cachedEdgesForExpect ; private AbstractStackNode [ ] sharedNextNodes ; private int location ; private final AbstractStackNode [ ] [ ] expectMatrix ; private final ArrayList < String > containerIndexMap ; private final int numberOfContainers ; private final int numberOfUniqueNodes ; public Parser ( char [ ] input , ParserStructure structure ) { } private boolean canEnter ( AbstractStackNode node ) { } private boolean canComplete ( AbstractStackNode node ) { } private AbstractStackNode updateNextNode ( AbstractStackNode next , AbstractStackNode node , AbstractNode result ) { } private boolean updateAlternativeNextNode ( AbstractStackNode next , AbstractStackNode node , AbstractNode result , IntegerObjectList < EdgesSet > edgesMap , ArrayList < Link > [ ] prefixesMap ) { } private void propagateReductions ( AbstractStackNode node , AbstractNode nodeResultStore , AbstractStackNode next , AbstractNode nextResultStore , int potentialNewEdges ) { } private void propagatePrefixes ( AbstractStackNode next , AbstractNode nextResult , int nrOfAddedEdges ) { } private void propagateEdgesAndPrefixes ( AbstractStackNode node , AbstractNode nodeResult , AbstractStackNode next , AbstractNode nextResult ) { } private void propagateEdgesAndPrefixesForNullable ( AbstractStackNode node , AbstractNode nodeResult , AbstractStackNode next , AbstractNode nextResult , int potentialNewEdges ) { } private void propagateAlternativeEdgesAndPrefixes ( AbstractStackNode node , AbstractNode nodeResult , AbstractStackNode next , AbstractNode nextResult , int potentialNewEdges , IntegerObjectList < EdgesSet > edgesMap , ArrayList < Link > [ ] prefixesMap ) { } private void updateEdges ( AbstractStackNode node , AbstractNode result ) { } private void updateNullableEdges ( AbstractStackNode node , AbstractNode result ) { } private void handleEdgeSet ( EdgesSet edgeSet , Link resultLink , int startLocation ) { } private void moveToNext ( AbstractStackNode node , AbstractNode result ) { } private void move ( AbstractStackNode node , AbstractNode result ) { } private void reduce ( ) { } private boolean findFirstStacksToReduce ( ) { } private boolean findStacksToReduce ( ) { } private void addTodo ( AbstractStackNode node , int length , AbstractNode result ) { } private void handleExpects ( EdgesSet cachedEdges , AbstractStackNode [ ] expects ) { } private void expandStack ( AbstractStackNode node ) { if ( ! ( canEnter ( node ) ) ) return ; if ( node . isMatchable ( ) ) { addTodo ( node , node . getLength ( ) , node . getResult ( ) ) ; } else if ( ! ( node . isExpandable ( ) ) ) { EdgesSet cachedEdges = cachedEdgesForExpect [ node . getContainerIndex ( ) ] ; if ( cachedEdges == null ) { cachedEdges = new EdgesSet ( 1 ) ; cachedEdgesForExpect [ node . getContainerIndex ( ) ] = cachedEdges ; AbstractStackNode [ ] expects = expectMatrix [ node . getContainerIndex ( ) ] ; if ( expects == null ) return ; handleExpects ( cachedEdges , expects ) ; } else { if ( ( cachedEdges . getLastVisistedLevel ( ) ) == ( location ) ) { stacksWithNonTerminalsToReduce . push ( node , cachedEdges . getLastResult ( ) ) ; } } cachedEdges . add ( node ) ; node . setIncomingEdges ( cachedEdges ) ; } else { EdgesSet cachedEdges = cachedEdgesForExpect [ node . getContainerIndex ( ) ] ; if ( cachedEdges == null ) { cachedEdges = new EdgesSet ( ) ; cachedEdgesForExpect [ node . getContainerIndex ( ) ] = cachedEdges ; AbstractStackNode [ ] listChildren = node . getChildren ( ) ; for ( int i = ( listChildren . length ) - 1 ; i >= 0 ; -- i ) { AbstractStackNode child = listChildren [ i ] ; int childId = child . getId ( ) ; AbstractStackNode sharedChild = sharedNextNodes [ childId ] ; if ( sharedChild != null ) { sharedChild . setEdgesSetWithPrefix ( cachedEdges , null , location ) ; } else { if ( child . isMatchable ( ) ) { int length = child . getLength ( ) ; int endLocation = ( location ) + length ; if ( endLocation > ( input . length ) ) continue ; AbstractNode result = child . match ( input , location ) ; if ( result == null ) continue ; <START_BUG> if ( ! ( canEnter ( node ) ) ) <END_BUG> continue ; child = child . getCleanCopyWithResult ( location , result ) ; addTodo ( child , length , result ) ; } else { child = child . getCleanCopy ( location ) ; stacksToExpand . push ( child ) ; } child . initEdges ( ) ; child . setEdgesSetWithPrefix ( cachedEdges , null , location ) ; sharedNextNodes [ childId ] = child ; } } if ( node . canBeEmpty ( ) ) { AbstractStackNode empty = node . getEmptyChild ( ) . getCleanCopy ( location ) ; empty . initEdges ( ) ; empty . addEdges ( cachedEdges , location ) ; stacksToExpand . push ( empty ) ; } } if ( ( cachedEdges . getLastVisistedLevel ( ) ) == ( location ) ) { stacksWithNonTerminalsToReduce . push ( node , cachedEdges . getLastResult ( ) ) ; } cachedEdges . add ( node ) ; node . setIncomingEdges ( cachedEdges ) ; } } private void expand ( ) { } public AbstractNode parse ( String start ) { } }

Fix:
if ( ! ( canEnter ( child ) ) )